Chemical vapor deposition is a well known technique and is known to provide coatings and films with properties superior to that obtained by other methods, such as sputtering or in situ pyrolysis of a pre-existing film. Chemical vapor deposition provides greater uniformity, adherence, and control of properties than do alternate coating methods. Pyrolysis of a non-volatile organic film on a surface is, in particular, inferior to chemical vapor deposition in these regards. Chemical vapor deposition as now practiced employs gaseous precursors and is often conducted at reduced pressures. Such reduced pressures are often necessary to achieve uniformity. The necessity to work at reduced pressures is a serious disadvantage in that it reduces production rates and requires relatively expensive equipment.
The production of coated refractory fibers is known, as taught, for example, in U.S. Pat. No. 3,428,519, issued Feb. 18, 1969 to Carl M. Zavanut and in U.S. Pat. No. 3,565,683, issued Feb. 23, 1971 to Charles R. Morelock. These and similar processes for the deposition of carbonaceous coatings require that a long continuous fiber be drawn through a heated reaction zone. Because of their small size, small particles and short fibers cannot be coated by these methods. And, if long continuous fibers are made by the methods taught by these patents and then chopped into smaller particles or short fibers, the resulting smaller particles or short fibers will not have substantially their entire surfaces coated with the carbonaceous coatings since the surfaces where the breaks occur will be fresh uncoated surfaces.
U.S. Pat. No. 4,510,077, issued Apr. 9, 1985, to Richard K. Elton, discloses the preparation of short glass fibers coated with a carbonaceous film by in situ pyrolysis of a coating chosen to pyrolize below its boiling or sublimation point. The method taught by Elton is specifically an in situ pyrolysis of a pre-existing film, and is not a chemical vapor deposition process.
The preparation of carbonaceous films on particles by pyrolysis of volatile organic vapors is known. For example, U.S. Pat. No. 3,651,386, issued Mar. 21, 1972, to Karl J. Youtsey, et al., discloses the preparation of coated refractory oxide particles by passing a volatile organic vapor through a stationary bed of such particles at a high temperature. U.S. Pat. No. 3,997,689, issued Dec. 14, 1976, to George L. Herbert, describes a process for coating refractory oxide particles in which the particles are maintained in a semi-fluidized state and a volatile organic vapor is passed through the semi-fluidized particles. Processes involving passing organic vapor through a stationary bed of particles are unlikely to provide a even amount of coating from one particle to the next since the organic vapor is depleted as it rises through the bed, and uneven gas flows are unlikely to occur. While the uniformity of such processes can be improved by maintaining the particles in a fluid or semi-fluid state, many desirable particles cannot be maintained in a fluid or semi-fluid state. In particular, particles such as short fibers that are not substantially spherical cannot be maintained in a fluid state. Also, some spherical particles, such as hollow microspheres, cannot be fluidized at the temperatures required for deposition of carbonaceous films because of the agglomerative properties of the particles. Difficulties and limitations involved in maintaining fluidized beds of hollow silica microspheres are described in, for example, Proceedings of the Conference on Chemical Vapor Deposition Fifth International Conference, Edited by J. M. Blocher, H. E. Hintermann and L. H. Hall, The Electrochemical Society, Princeton, New Jersey, 1975, in an article entitled "Microspherical Laser Targets by CVD" by William J. McCreary, pages 714-725.
The prior art neither teaches nor suggests that chemical vapor deposition processes may be carried out by heating, in the substantial absence of oxygen, an intimate mixture of a plurality of small particles and an appropriately chosen solid precursor. Indeed, the prior art would lead one of ordinary skill in the chemical vapor deposition art to avoid the use of such an intimate mixture as a method for practicing chemical vapor deposition.